1. Field of the Invention
The present invention relates to a signal-synthesizing method, and more particularly, to a nonlinear overlap method for time scaling.
2. Description of the Prior Art
Due to the dramatic progress in electronic technologies, an AV player such as a Karaoke can provide more and more amazing functions, such as audio clean-up, dynamic repositioning of enhanced audio and music (DREAM), and time scaling. Time scaling (also called time stretching, time compression/expansion, or time correction) is a function to elongate or shorten an audio signal while keeping the pitch of the audio signal approximately unchanged. In short, time scaling only adjusts the tempo of an audio signal.
In general, an AV player performs time scaling with one of the three following methods: Phase Vocoder, Minimum Perceived Loss Time Expansion/Compression (MPEX), and Time Domain Harmonic Scaling (TDHS). Phase Vocoder transforms an audio signal into a complex Fourier representation signal with Short Time Fourier Transform (STFT) and further transforms the complex Fourier representation signal back to a time scaled audio signal corresponding to the original audio signal with interpolation techniques and iSTFT (inverse STFT). MPEX is a method researched and developed by Prosoniq for simulating characteristics of human hearing, similar to an artificial neural network. MPEX records audio signals received for a predetermined period and tries to “learn” the audio signals, so as to either elongate or shorten the audio signals. TDHS is one of the most popular methods for time scaling. TDHS first establishes an autocorrelogram of a first audio signal, the autocorrelogram consisting of a plurality of magnitudes, and then delays the first audio signal by a maximum index corresponding to a maximum magnitude, a largest magnitude among all of the magnitudes of the autocorrelogram, to form a second audio signal, and lastly synchronizes and overlap-adds (SOLA) the first audio signal to the second audio signal to form a third audio signal longer than the first audio signal.
In a computer system, the autocorrelogram is usually established by a digital signal processing (DSP) chip designed to manage complex mathematic calculation such as convolution and fast Fourier transform (FFT). However, a process by the DSP chip to synthesize the third audio signal from the first and second audio signals is tedious and sometimes unnecessary.